The use of lasers for ophthalmic surgery is well established. There has been constant variation and improvement to the delivery systems for directing the treatment laser beam to the region of the eye that is to receive the treatment. A high degree of pointing accuracy is required because of the sensitivity of the eye and the delicate nature of the surgery. A standard approach is to use an instrument that includes a conventional slit lamp microscope for use by the surgeon to view the eye, an aiming laser, and the treatment laser which is arranged to be coincident with the aiming laser.
The treatment laser beam can be delivered according to a number of different scenarios. An early approach was to deliver the treatment laser beam via the slit-lamp illumination path. This approach is described in U.S. Pat. No. 4,765,336, assigned to Carl-Zeiss-Stiftung. Variations on the same principle can be found in U.S. Pat. No. 5,000,560, assigned to Haag-Streit, and U.S. Pat. No. 5,425,729, assigned to Kowa Company Ltd and Coherent Incorporated. The problem with this approach is that the position of the treatment laser beam moves with the position of the slit lamp illumination. This is undesirable since it limits the ability of the surgeon to adjust the treatment laser beam completely independently from the slit lamp illumination.
A second approach is to deliver the treatment laser beam via a dichroic beam splitter that is placed in front of the viewing microscope objective lens. Reference may be had to U.S. Pat. Nos. 5,226,903 and 5,954,711, both assigned to Nidek Co Ltd, for a description of this approach. Both of these patents describe the use of a fiber-optic delivery system for bringing the treatment laser beam and the aiming beam to the viewing microscope. This type of system is typically an external accessory that is attached to the top of the slitlamp microscope, normally as a retrofit. This configuration suffers from a reduction in the working distance between the slitlamp objective lens and the patient's eye due to the positioning of the beam splitter. The beam splitter also introduces astigmatic aberrations into the viewing and illumination paths, reduces the sharpness of the viewing area, restricts the laser wavelengths that can be used, adds discolouration and increases cost. In addition, because the delivery system is fitted by the user as an accessory, correct operation is dependant on the ability of the user to correctly align it. The introduction of additional optical surfaces outside of the slit lamp microscope housing also greatly increases problems with dust, scratches, breakage of optics and ghost reflections. The fiber cable is exposed and prone to potential overbending and breakage.
The wavelength restrictions and astigmatic aberrations can be minimized by delivering the treatment laser beam via a broadband mirror positioned between the viewing paths of the left and right sides of the stereo viewing microscope. Such an arrangement is described in U.S. Pat. No. 5,002,386, assigned to G Rodenstock Instruments GmbH. The arrangement fails to address the remaining shortcomings listed above.
The prior art ophthalmic laser systems each suffer from one or more disadvantages as discussed above. The use of a fiber-optic delivery system offers advantages but external positioning of the fiber subjects the laser to likely damage. A better arrangement is required.